The present invention relates to the production of hydrate-forming phosphates, and more particularly to novel moisturized, crystalline compositions of hydrate-forming phosphates and methods for production thereof.
For many years, a heavy demand for sodium tripolyphosphate (STP) has resulted from its many uses, particularly as a detergent builder and in the processing of food products. However, conventional STP compositions have several drawbacks. For example, currently available STP compositions do not dissolve in water as rapidly as is desired, that is, their rates of solution are not as high as desired. The rate of solution of an STP composition has been linked to the composition's rate of hydration. Moreover, the powder form of STP compositions tends to cake, creating serious problems, as discussed below.
Conventionally, the manufacture of STP involves a process such as that disclosed in U.S. Pat. No. 3,233,967 to Shen and calcining the tripolyphosphate so produced at a temperature of between 200.degree. C. and 600.degree. C. Calcining at temperatures between about 250.degree. C. and about 375.degree. C. results in the production in what is known as Phase II STP. Relatively higher calcining temperatures, i.e., those in excess of 417.degree. C., and preferably in excess of 550.degree. C., produce what is known as Phase I STP. Phase II STP has a lower rate of hydration than Phase I, and so a lower rate of solution, but has flow properties superior to Phase I STP and, due to the lower calcining temperature required, it is far less expensive to manufacture than Phase I STP.
Commercially available STP ordinarily comprises a blend of the two phases. Blending of the two phases to various relative proportions allows the characteristics of the resulting STP blend to be varied to some degree. Thus, blends of Phase I STP and Phase II STP have been utilized as a compromise of the desirable and undesirable characteristics of each STP phase. Ordinarily, 20% by weight to 50% by weight of commercial STP is found to be Phase I STP.
However, since such mixtures are compromises between the advantages as well as the disadvantages of two phases of STP, the STP blends have several drawbacks. It is desired that the blends more readily hydrate and dissolve in water, that is, that they have higher rates of hydration in shorter periods of time. While Phase I STP has a higher rate of hydration than does Phase II STP, the rate of hydration even of Phase I STP is not as high as desired. In addition, Phase I STP tends to cake, and this tendency to cake and form clumps results in generally poor flow characteristics. During shipment, oftentimes large volumes of STP cake so as to form in the cargo vessel a large, hardened block of STP, the removal of which requires an expensive process, commonly involving pneumatic hammers. Moreover, due to the higher calcining temperature required for the production of Phase I STP, Phase I STP tends to be far more expensive, as much as 40% more expensive, to manufacture than Phase II STP. Some manufacturers add a small amount of potassium to aid in the conversion of Phase II STP to Phase I STP.
More specifically, crystalline STP compositions are generally useful in either powder or granular form, each form having drawbacks peculiar to that form, as will be discussed below. In the powder form, generally 60% to 70% of the particles are smaller than 270 mesh, and essentially all the particles are smaller than 60 mesh. On the other hand, in granular form, at least 40% of the particles exceed 60 mesh, while all particles (except for a trace) exceed 100 mesh. Both STP composition forms are useful in detergents as builders. As a builder, STP tends to chelate calcium and magnesium ions found in tap water, thereby softening the water and aiding the detergent's cleaning action. In application as a builder, it is desirable that both STP composition forms dissolve and hydrate quickly and completely. Accordingly, both forms of the composition should have high rates of hydration in short periods of time, and therefore, high rates of solution. Moreover, it is desired that the rates of hydration, and thus, rates of solution, of STP compositions be more consistent in order to achieve more predictable results. In other words, it is desired that the rates of hydration be more consistent from sample to sample of STP composition.
Powder STP compositions are used in ordinary household detergents. Typically, water is added to such compositions in order to increase the rate of hydration. A high rate of hydration is particularly desirable in the processing of materials for formulating detergents since a high rate of hydration indicates that heat will be given off during the processing, thereby aiding the processing. Aside from the desirability of having a high rate of hydration in a short period of time, it is important that the powder remain free-flowing and resist caking. However, the addition of water for increasing the rate of hydration tends to exacerbate caking. As described above, caking of STP compositions during storage and shipment creates severe problems.
Granular STP compositions are typically used in a dry mix detergent for commercial dishwashing by agglomerating granular STP with other detergent ingredients, such as soda ash and silicates into a solid block that is placed in the commercial washing machine. Thus, it is important that the STP dissolve at rate near that of the other ingredients in the detergent block. Accordingly, use of Phase II STP in such applications suffers disadvantages because Phase II STP has a low rate of hydration and the other detergent block ingredients tend to dissolve before the STP. For at least two reasons, use of Phase I STP or a mixture of the two phases does not adequately solve this problem. First, as explained, Phase I STP is relatively expensive to manufacture. Second, it is desired that the STP have a rate of hydration higher even than that of conventional Phase I STP.
In attempts to solve the problems with granular Phase II STP, it has been found that the addition of water may produce Phase II STP with a rate of solution equivalent to that of Phase I STP. However, several problems still remain. Addition of water in an amount necessary to produce such a rate of solution in Phase II STP results in a STP composition which is difficult to process, and therefore expensive to manufacture. Moreover, it is desired that the STP have a rate of solution still higher than that achieved in such a manner. Thus, an STP composition is desired which has a higher rate of solution, and which requires less water to achieve it.
For decades, the industry has attempted to solve the severe drawbacks of the conventional STP compositions. These attempts, however, have met with little success. As a result, while STP is important to several industries, the disadvantages have proved troublesome and costly and the use and sales of STP have suffered as a result.
Problems similar to those attending to STP compositions are encountered with other compositions of hydrate-forming phosphates, that is, other phosphates which are capable of forming hydrates, particularly sodium pyrophosphate and trisodium phosphate, also tend to cake in the presence of water. Further, as with STP, it is desired that these other phosphates also have higher rates of solution.
U.S. patents relating to phosphate compositions and detergents include U.S. Pat. No. 3,233,967 to Shen, issued Feb. 8, 1966; U.S. Pat. No. 3,244,478 to Stahlheber, issued Apr. 5, 1966; U.S. Pat. No. 3,248,330 to Feierstein et al., issued Apr. 26, 1966; U.S. Pat. No. 3,397,947 to Shaver, issued Aug. 20, 1968; U.S. Pat. No. 3,397,948 to Mesmer, issued Aug. 20, 1968; U.S. Pat. No. 3,426,440 to Shen et al., issued Feb. 11, 1969.